Prevention of hydrogen embrittlement of high-strength steel by hydrazine compositions

ABSTRACT

Delayed failure of high-strength steel alloys exposed to compositions containing hydrazine is prevented by addition of potassium hydroxide to the composition in an amount at least sufficient to neutralize acidic impurities. The removal of the acidic impurities eliminates evolution of hydrogen and thus avoids hydrogen embrittlement of the high-strength steel alloys.

United States Patent [1 1 Fletcher et a1.

1 1 PREVENTION OF HYDROGEN EMBRI'ITLEMENT OF HIGH-STRENGTH STEEL BYHYDRAZINE COMPOSITIONS [76] Inventors: James C. Fletcher, Administratorof the National Aeronautics and Space Administration, with respect to aninvention by Leonard Weber, Noithridge, Calif.

22 Filed; Sept. 28, 1973 211 Appl. No.1401.921

1 1 Nov. 11,1975

6/1973 Rigshy 413/4117 7/1973 Lamh et a1 149/36 X OTHER PUBLICATIONSAudrieth et al. The Chemistry of Hydrazine." pp. 140, 141. l45148. 213and 214. John Wiley & Sons. Inc. (1951) New York.

Primary E.\'uminerBenjamin Rv Padgett Assistant Examiner-E. A. MillerAttorney, Agent, or Firm-Monte F. Mott; Wilfred Grifka; John R. Manning[57] ABSTRACT Delayed failure of high-strength steel alloys exposed tocompositions containing hydrazine is prevented h; addition of potassiumhydroxide to the composition in an amount at least sufficient toneutralize acidic impurities. The removal of the acidic impuritieseliminates evolution of hydrogen and thus avoids hydrogen embrittlementof the high-strength steel alloys.

7 Claims, N0 Drawings PREVENTION OF HYDROGEN EMBRITTLEMENT OFHIGH-STRENGTH STEEL BY HYDRAZINE COMPOSITIONS ORIGIN OF THE INVENTIONThe invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Aeronautics and Space Act of I958, Public Law 83-568 (72 Stat.435; 42 USC 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates to an improved hydrazine composition and, moreparticularly, to such a composition capable of long-term contact withhighstrength steel surfaces.

2. Description of the Prior Art Steel alloys such as Type 410 StainlessSteel have been found susceptible to delayed failure when ex posed tocompositions containing acidified hydrazine. Fractography indicates thatsuch failures are similar to known hydrogen embrittlement failures andstress corrosion failures in 410 Stainless Steel. It was found thatchemical reactions can occur which release hydrogen at the steelsurface. The released hydrogen penetrates the steel and may produceembrittlement or stress corrosion rendering the steel unsuitable forspace vehicle tankage and long-term storage.

High-strength steels are highly desirable for space vehicle tankagebecause of their high-strength/weight ratio, stiffness and ease ofmanufacture. Such steels, however, are unsuitable for storage tanks forliquid fuei compositions containing hydrazine such as Aerozine- 50 inview of the demonstrated incompatibility with acidified hydrazine. Thealternative is to fabricate tanks from materials which are notsusceptible to hydrogen embrittlement. Aluminum alloys would probablymeet these requirements. However, designs to meet required weightlimitations would introduce additional complexities and costs. Inaddition, aluminum tanks are more costly to fabricate because of thegreater difficul- 3 N,H N +4 NH Small amounts of hydrogen are also saidto be produced in this decomposition but the reaction pathway isobscure. In any event, the amount of hydrogen produced in such autolysisreactions is negligible compared with the large amounts produced in thereaction between steel and the NgHg ion.

This reaction is analogous to the aqueous reaction between hydrogen ionand steel.

2 H O Fe 2 H O Fe H;-

The N l-l ion arises from the presence of acidic substances dissolved inthe fuel, in the same way that H O is produced from acids dissolved inwater:

where HA is a molecule of any acid.

In Acrozine-SO, the most likely acidic species is carbazic acid, whichis formed by reaction of atmospheric carbon dioxide with hydrazine:

Other possible sources of H,H,, are ammonium chloride and hydraziniumchloride, both of which may be present in trace amounts.

It should be emphasized that atmospheric carbon dioxide is not the onlypossible source of acidic impurities in hydrazine-base fuels. Forexample, halogenated solvents will react with hydrazine to yieldhydrazinian salts which act as acids according to the followingreactIon:

where X is halogen and R is an organic group. Solvents which fall intothis category are Freon, trichloroethylene, methylene dichloride and thelike.

Thus, it would appear that an obvious solution to counter the hydrogenproducing reactions would be the exposure of the hydrazine fuel to basicsubstances to remove and neutralize the acidic impurities. However. whenbasic materials such as barium oxide or sodium hydroxide were added tothe composition, they were found to form a precipitate which is quiteobjectionable in respect to contaminating components with small orifices(i.e., valves, injections ports and control systems) unless the treatedcomposition is filtered before storage and prior to use to eliminate theprecipitate. Filtering is a very expensive operation and it would bedesirable to eliminate it. Furthermore, it would be desirable to havethe neutralizing agent present in excess within the composition so thatacidic impurities such as carbon dioxide or hydrochloric acid can becontinuously neu tralized preventing the Aerozine-SO from attacking anddegrading the steel (i.e., 410). The excess neutralizing agents such asbarium oxide or sodium hydroxide present in the storage tank would reactwith further amounts of carbon dioxide to form insoluble carbonateswhich must be filtered and removed before the composition could be sentthrough the valves or if not properly filtered, the precipitate couldclog orifices re sulting in catastrophic failures.

Austenitic steels and especially weld seams are very susceptible tostress-environment failures when exposed under sustained load tohydrazine liquid fuels.

Experience has shown that fabricated hardware sometimes containundetected flaws, frequently in welds, and residual stresses resultingfrom manufacturing operations. In the presence of an aggressive chemicalenvironment, such flaws may grow, causing leaks or catastrophicfracture. Hydrogen permeation testing of 410 Stainless Steel membraneswith several surface treatments demonstrated that the treatments failedto completely suppress hydrogen flux generation. The treatmentsconsisted of pickling alone or in combina tion with distilled waterrinses or further in combination with aqueous ammonium fluoride,S-hydroxy quinoline, perfluoro-octanoic acid, stearic acid orperfluoro-decyl-l-sulfonic acid.

Atomic absorption spectrometry for precipitates suspended in Aerozine-SOfollowing treatment with sieved V4 inch granular barium oxide revealedapproximately 3 ppm barium in the supernatant liquid. Filtration with amicron woven stainless steel wire mesh filter reduced the barium contentto 0.9 ppm and further filtration using a 1.0 micron Teflon filter wasrequired to reduce the barium level to 0.6 ppm.

The specifications for the second stage of the Delta rocket entails afirst use of a propellant combination of Aerozine-St] and N 0, with Type410 Stainless Steel tankage. The available published literature does notreveal data establishing compatibility of this fuel combi nation with410 Stainless Steel welds. The literature does indicate that 410 parentmetal catalyzes decomposition of Aerozine-SO and the two materials whencombined in a glass vial. However, sustained load compatibility testshave not been performed with 410 and Aerozine-50. A program wasconducted to determine the compatibility of Aerozine-50 and N 0,, with410 Stainless Steel welds under stress, primarily to determine whetherAerozine-SO and/or N 0, would produce flaw extension in welded 410Stainless Steel stressed in tension.

The program determined that welded specimens of 410 Stainless Steelfailed in sustained load within 3 to 30 days exposure to Aerozine-SO. N0 specimens survived the 30day exposure with no evidence of flaw growth.A number of parameters were investigated to explore their individual.and collective effects on time to failure. The parameters included:acidic contaminant condition of the Aerozine-SO, test temperature range,method of achieving test temperature range, specimen fuel containermaterial, specimen surface condition, and the number of load cyclessupplied to the specimens. An analysis of the data leads to theconclusion that the single most significant factor incluencing flawgrowth rates and failures is the condition of the fuel.

SUMMARY OF THE INVENTION In accordance with the invention, it has beendiscovered that addition of potassium hydroxide to hydrazinecontainingfuel compositions that are in contact with austenitic steels controlsthe dissociation constant of hydrazine significantly to reduce andeliminate H ion, i.e., acidity, without forming a precipitate, thusobviating filtration. Both potassium hydroxide and its neutralizationreaction products are soluble in hydrazine fuels.

The potassium hydroxide containing hydrazine fuel composition inaccordance with the invention prevents corrosion of steel duringstorage, inhibits decomposition and loss of hydrazine from the storedfuel and permits long-term storage in welded, high-strength, austeniticsteel storage tanks under load without fear or danger of acidic impurityinduced hydrogen embrittlement or catastrophic fracture.

The acidic impurities may be present in the fuel composition asmanufactured or may be absorbed from the atmosphere during storage orcontaminated by haloge nated solvents. The potassium hydroxide is addedin a minimum concentration necessary to achieve neutrality. For example,Aerozine-SO containing 200 ppm CO requires about 260 to 400 ppmpotassium hydroxide for neutralization and stabilization. This is a 1/1molar ratio and reaction.

The potassium hydroxide may be added in an excess amount over thestoichiometric amount necessary for neutralization in order toaccommodate and neutralize further amounts of acidic impurities such asCO which are abosrbed from the ambient environment during extendedperiods of storage. The potassium hydroxide may suitably be added in anamount of 20 to 200 ppm over the amount necessary for neutralization ofthe as manufactured acidic impurities capable of causing evolution ofhydrogen in the presence of austenitic steel surfaces. Typically,potassium hydroxide is added to the fuel in an approximate concentrationof 200 ppm to neutralize the acid impurities and/or to maintain theequilibrium in direction of N H stability.

The invention is generally applicable to hydrazinecontainingcompositions such as anhydrous hydrazine, unsymmetrical dimethylhydrazine (UDMH), mixtures thereof such as Acrozine-50 (At-50), ormixtures with oxidizers such as N 0... Aerozine-SO has a generalcomposition comprising about 47.76% UDMH, 51.76% bydrazine and 0.36%water. The stress corrosion, hydrogen embrittlement of the compositionin presence of austenitic stainless steel was tested by analyzing fordiffusion of hydrogen through a steel membrane. The detection techniquedepends on the observation that hydrogen generated by the reaction ofacid at a metal surface diffuses into the metal. This diffusioneventually results in permeation of some hydrogen entirely through thespecimen. With a suitable device this hydrogen can be detected as apermeation flux.

The device utilized for testing consisted of two compartments separatedby a 0.25 inch steel membrane. This membrane becomes a working electrodeof a potentiostat. One of the compartments is filled with 1.0N sodiumhydroxide solution. The reference electrode and the auxiliary electrodeof the potentiostat are immersed in the sodium hydroxide and themembrane is brought to +100 MVsce. After the membrane has been polarizedto equilibrium, the test solution is placed in the other compartment.Hydrogen generated at the membrane surface diffuses through the membraneand is oxidized to H ion by the +100 MVsce. Under the influence of thepotential, the l'l ions migrate to the auxiliary electrode where theyare discharged, producing a current flow in the auxiliary circuit. Thecurrent is detected as an IR drop across a standard resistor and isdisplayed on a recorder.

EXAMPLE 1 Barium oxide was added to 300 grams of Aerozine- 50 untilevolution of hydrogen was terminated. The composition was filtered toremove 0.6 ppm of solids.

Carbon dioxide was then generated by combining 0.1 grams of Na CO and HThe evolved CO, was trapped and added to 300 grams of Aerozine-50 in anamount of 200 ppm. 51 ml of the CO containing Aerozine-50 compositionwas treated with 0.1 ml of a 20 g/ cc KOH solution. The Aerozine-S0 wasfree of carbon dioxide and no hydrogen flux developed. The

composition was clear and no opalcsccnce could be seen.

EXAM PLE 2 Example 1 was repeated substituting 0.1 ml of a g/l00 cc NaOHsolution for the KOH additive. Although the A-50 was free of carbondioxide and there was no hydrogen flux, an opalescent precipitatedeveloped after 2 minutes.

it is to be understood that only preferred embodiments of the inventionhave been described and that numerous substitutions, alterations andmodifications are all permissible without departing from the spirit andscope of the invention as defined in the following claims What isclaimed is:

l. A stabilized composition compatible with highstrength steel surfacesconsisting essentially of a hydrazine containing material selected fromthe group consisting of substantially anhydrous hydrazine, unsymmetricaldimethyl hydrazine. and mixtures thereof, said material containingacidic impurities, and a minor amount of potassium hydroxide at leastsufficient to neutralize said impurities 2. A composition according toclaim 1 in which potassium hydroxide is present in an amount of 260 to400 ppm for 200 ppm of CO impurity.

3. A composition according to claim 1 in which the potassium hydroxideis present in an amount of 20 lo 200 ppm over the amount necessary tostoichiometrically neutralize the impurities present in the compositionat the time of addition ofthe potassium hydroxide.

4. A method of stabilizing an acid impurity contaminated hydrazinecontaining composition from evolu tion of hydrogen without the formationofa precipitate. said hydrazine containing composition selected from thegroup consisting of substantially anhydrous hydrazine, unsymmetricaldimethyl hydrazine. and mixtures thereof, comprising;

adding potassium hydroxide to said composition in a minor amount atleast sufficient to neutralize said impurities.

5. A method according to claim 4 further comprising the step of placingsaid stabilized composition in contact with the surface of austeniticsteel 6. A method according to claim 5 in which the potassium hydroxideis added in an amount of 260 to 400 ppm for 200 ppm of CO impurity.

7. A method according to claim 5 in which the potassium hydroxide isadded in an amount of 20 to 200 ppm over the amount necessary tostoichiometrically neutralize the impurities present in the compositionat the time of addition of the potassium hydroxide.

1. A STABILIZED COMPOSITION COMPATIBLE WITH HIGH-STRENGTH STEEL SURFACESCONSISTING ESSENTIALLY OF A HYDRAZINE CONTAINING MATERIAL SELECTED FROMTHE GROUP CONSISTING OF SUBSTANTIALLY ANHYDROUS HYDRAZINE, UNSYMMETRICALDIMETHYL HYDRAZINE, AND MIXTURES THEREOF, SAID MATERIAL CONTAININGACIDIC IMPURTIES, AND A MINOR AMOUNT OF POTASSIUM HYDROXIDE AT LEASTSUFFICIENT TO NEUTRALIZE SAID IMPURITIES.
 2. A composition according toclaim 1 in which potassium hydroxide is present in an amount of 260 to400 ppm for 200 ppm of CO2 impurity.
 3. A composition according to claim1 in which the potassium hydroxide is present in an amount of 20 to 200ppm over the amount necessary to stoichiometrically neutralize theimpurities present in the composition at the time of addition of thepotassium hydroxide.
 4. A method of stabilizing an acid impuritycontaminated hydrazine containing composition from evolution of hydrogenwithout the formation of a precipitate, said hydrazine containingcomposition selected from the group consisting of substantiallyanhydrous hydrazine, unsymmetrical dimethyl hydrazine, and mixturesthereof, comprising; adding potassium hydroxide to said composition in aminor amount at least sufficient to neutralize said impurities.
 5. Amethod according to claim 4 further comprising the step of placing saidstabilized composition in contact with the surface of austenitic steel.6. A method according to claim 5 in which the potassium hydroxide isadded in an amount of 260 to 400 ppm for 200 ppm of CO2 impurity.
 7. Amethod according to claim 5 in which the potassium hydroxide is added inan amount of 20 to 200 ppm over the amount necessary tostoichiometrically neutralize the impurities present in the compositionat the time of addition of the potassium hydroxide.